I've got suppliers who I can send a difficult part to and know that I'm going to get exactly what I expect, faster and cheaper than just about anyone else. It took a few years to get to that point, but these few vendors make it really hard to go with anyone else, much to the chagrin of the sourcing team who rightly recognize it a risk to rely on just a few suppliers.
Once you get to a certain type of supplier you end up running into the problem where their processes are such that they won't do anything without you clearly documenting it. They simply refuse to make any assumptions on your behalf. They can be so frustrating when you are used to the other way of doing it. I simply cannot answer some questions because I'm so used to my other suppliers just doing it correctly and haven't ever asked about it.